EFFECTS OF AUSTENITE GRAIN-SIZE AND COOLING RATE ON WIDMANSTATTEN FERRITE FORMATION IN LOW-ALLOY STEELS

Deformation dilatometry is used to simulate the hot rolling of 0.20 pc t C-1.10 pct Mn steels over a product thickness range of 6 to 170 mm. In addition to a base steel, steels with additions of 0.02 pct Ti, 0.0 6 pct V, or 0.02 pct Nb are included in the study. The transformation behavior of each steel is explored for three different austenite grain sizes, nominally 30, 55, and 100 mum. In general, the volume fraction of Widmanstatten ferrite increases in all four steels with increasing austenite grain size and cooling rate, with austenite grain size havi ng the more significant effect. The Nb steel has the lowest transforma tion temperature range and the greatest propensity for Widmanstatten f errite formation, while the amount of Widmanstatten ferrite is minimiz ed in the Ti steel (as a result of intragranular nucleation of polygon al ferrite on coarse TiN particles). The data emphasize the importance of a refined austenite grain size in minimizing the formation of a co arse Widmanstatten structure. With a sufficiently fine prior austenite grain size (e.g., less-than-or-equal-to 30 mum), significant amounts of Widmanstatten structure can be avoided, even in a Nb-alloyed steel.